1. Field of the Invention
The present invention relates to an optical tomograph that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement.
2. Description of the Related Art
Conventionally, optical tomographs that utilize OCT measurement are used to obtain optical tomographic images of living tissue. In these optical tomographs, a low coherence light beam emitted from a light source is divided in to a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected or backscattered by a measurement target when the measuring light beam is irradiated onto the measurement target, is combined with the reference light beam. Tomographic images are obtained, based on the intensity of a coherent light beam obtained by combining the reflected light beam and the reference light beam. Hereinafter, the light beam, which is reflected or backscattered by the measurement target, will be collectively referred to as a “reference light beam”.
OCT measurement can be roughly divided into two types, TD-OCT (Time Domain Optical Coherence Tomography) and FD-OCT (Fourier Domain Optical Coherence Tomography). In TD-OCT measurement, the intensity of the interference light beam is measured while changing the optical path length of the reference light beam. Thereby, intensity distributions of the reflected light beam corresponding to measuring positions in the depth direction of the measurement target (hereinafter, referred to as “depth positions”) are obtained.
On the other hand, in FD-OCT measurement, the optical path lengths of the reference light beam and the signal light beam are not changed. The intensity of the interference light beam is measured for each spectral component thereof, and frequency analysis, such as Fourier transform, is administered on the obtained spectral interference intensity signals. Thereby, intensity distributions of the reflected light beam corresponding to the depth positions of the measurement target are obtained. FD-OCT measurement has been gathering attention recently as a method that enables high speed measurement, due to the mechanical scanning associated with TD-OCT measurement being obviated.
Optical tomographs that perform SD-OCT (Spectral Domain Optical Coherence Tomography) measurement and optical tomographs that perform SS-OCT (Swept Source Optical Coherence Tomography) measurement are two types of optical tomographs that employ FD-OCT measurement. In an SD-OCT optical tomograph, a wide band low coherence light beam is emitted from an SLD (Super Luminescent Diode), an ASE (Amplified Spontaneous Emission) light source, or a white light source. The wide band low coherence light beam is divided into a measuring light beam and a reference light beam by a Michelson interferometer or the like. Thereafter, the measuring light beam is irradiated onto a measurement target, and a reflected light beam reflected by the measurement target is caused to interfere with the reference light beam. The interference light beam formed thereby is spectrally decomposed into each frequency component by a spectrometer, and the intensity of each frequency component of the interference light beam is measured by a detector array, in which elements such as photodiodes are provided in an array. A computer administers Fourier transform on the obtained spectral interference intensity signals, to obtain a tomographic image.
Meanwhile, an SS-OCT optical tomographs utilizes a light source that periodically sweeps the frequency of a laser beam. Reflected light beams of each wavelength are caused to interfere with reference light beams of each wavelength. Temporal waveforms of signals corresponding to the temporal variations in the frequency of the laser beam are measured, and a computer administers Fourier transform on the obtained spectral interference intensity signals, to obtain a tomographic image.
In order to obtain higher resolution and higher image quality tomographic images with an OCT apparatus, it is necessary for the wavelength band of light emitted by a light source to be wide, and to increase data points corresponding to the wide wavelength band. However, SD-OCT apparatuses commonly detect each wavelength of interference light beams with detector arrays, in which elements such as photodiodes are provided in an array. Therefore, the number of data points is limited by the number of elements provided in the detector array. If the number of elements of the detector array is to be increased in order to increase the number of data points, cost increases, productivity decreases, and measurement rates deteriorate, which is not favorable. In contrast, the number of data points can be increased in SS-OCT apparatuses by increasing the sampling frequency of a circuit that converts photoelectric current from a detector to digital values, assuming that the frequency sweeping period of the light source is constant. Therefore, the number of data points can be increased at low cost, while maintaining measurement rates in SS-OCT apparatuses.
In each type of OCT measurement described above, it is known that the use of a measuring light beam having a wide spectral width improves spatial resolution (refer to Japanese Unexamined Patent Publication No. 2002-214125). Japanese Unexamined Patent Publication No. 2002-214125 discloses a method for widening the spectral width of a measuring light beam, in which a plurality of light sources that each emit light beams having a different spectral band are used, and an optical integrator integrates the light beams emitted from the plurality of light sources, to obtain a single light beam.
With respect to SD-OCT measurement, Japanese Unexamined Patent Publication No. 2001-264246 discloses a method in which light beams, which are emitted by a plurality of gain media that have overlapping wavelength bands, are combined to form a continuous spectrum. With respect to SS-OCT measurement, Japanese Unexamined Patent Publication No. 2006-047264 discloses a configuration in which light beams emitted from a plurality of wavelength scanning light sources (each constituted by a gain medium and a wavelength selecting element) are combined. As another example, U.S. Pat. No. 6,665,320 discloses a configuration in which light beams emitted from a plurality of gain media are combined, and a single wavelength selecting element controls the plurality of gain media.
As described above, light beams emitted from a plurality of light sources are combined in order to obtain high spatial resolution. However, if the plurality of light beams having different wavelengths are simultaneously irradiated onto a measurement target, the interference data becomes mixed and undetectable, because the detector is constituted by single elements in conventional SS-OCT apparatuses.
For this reason, the apparatuses disclosed in Japanese Unexamined Patent Publication No. 2006-042764 and U.S. Pat. No. 6,665,320 are configured such that a light beam that enters a detector at a given time is of a single wavelength by controlling the light source or by providing switching elements. This configuration enables the use of a wide band measuring light beam, but it takes time to irradiate all of the wavelength bands of the measuring light beam, and as a result, a problem arises that the measurement rate decreases.